[sanitizer] Support Intel CET

1. Include <cet.h> in sanitizer_common/sanitizer_asm.h to mark Intel CET
support when Intel CET is enabled.
2. Add _CET_ENDBR to function entries in assembly codes so that ENDBR
instruction will be generated when Intel CET is enabled.

Reviewed By: dvyukov

Differential Revision: https://reviews.llvm.org/D111185